1. Field of the Invention
The invention relates to foaming or frothing a beverage such as milk for espresso type coffee drinks, hot chocolate, and the like by counter rotating paddles immersed within the milk where the paddles principally are formed of a screen or mesh.
2. Background Information
Espresso is strong coffee brewed by forcing steam under pressure through darkly roasted, finely ground coffee beans. This form of brewing can produce a thin layer of creamy, dark beige froth on the coffee's surface. Because Espresso is so strong, this rich, complex flavored beverage is served in a small two to three ounce cup known as a demitasse.
To quell the strong taste, some who partake in espresso add sugar. Alternatively, the espresso coffee may be mixed or topped with steamed milk or cream to form espresso based beverages such as cappuccinos, lattes, mochas and the like. The steamed milk forms a froth that adds flavor and texture to the espresso coffee as well as serves as a garnish. Espresso is popular in Europe whereas cappuccino and lattes have become popular in the United States.
Conventionally, the milk for espresso based beverages is warmed and frothed through superheated water vapors maintained at high pressure. A variety of machines have been patented, most seeking to overcome the inconsistent nature of steaming milk. See, for example, U.S. Pat. Nos. 4,960,042, 5,335,588, 5,423,245, 5,464,574, 5,738,002, and 5,862,740.
Generally, each steam producing espresso machine has a cavity in which water is turned to steam and placed under pressure. A valve-controlled steam wand is coupled to the cavity at one end and has a venturi jet at the other end. The wand extends from the machine and into a cup of milk. As the valve opens, the compressed steam expands as both water vapor and air into the milk. This rapidly raises the temperature of the fats in the milk and causes the milk to foam. The problem with this technique is that the water undesirably modifies the taste of the froth milk. Moreover, the quality of frothed milk is inconsistent from one application to another application.
There exists techniques to whip milk into a warm froth without the addition of hot water vapors. One technique is to direct the milk into a hard surface, thus causing the milk to rapidly change directions so as to expand into a froth. For example, U.S. Pat. No. 4,537,323 relates to an impeller having low vertical profile blade disposed within dairy based drinks where the impeller is driven at 4,000 Revolutions Per Minute (RPM) by external magnets. The high speed and low vertical profile of these blades forces the milk to rapidly change directions into surrounding air so as to produce fine, uniform bubbles within the milk. As another example, the device of U.S. Pat. No. 4,620,953 first heats milk through a heat exchanger and directs the hot milk through an adjustable-gap venturi into a stationary hole partially filled by cone. The cone provides an impact surface on which the milk impacts and rapidly changes direction. U.S. Pat. Nos. 4,949,631 and 5,759,604 operate similarly.
A technique to control a froth within a liquid without the addition of hot water vapors is to pass the liquid through stationary screens. For example, U.S. Pat. No. 5,151,199 teaches passing instant coffee through stationary screens to reduce the bubbles formed in instant coffee from large coarse bubbles to bubbles that comprise a fine, creamy froth. U.S. Pat. No. 5,738,002 teaches passing milk through a stationary frother nozzle having a plurality of holes that serve to aerate and cause frothing of the milk passing therethrough.
Another technique to whip milk into a warm froth without the addition of hot water vapors is to manually move coils, screens or meshes in a random or vertical direction through the milk. For example, U.S. Pat. No. 5,482,367 teaches manually rotating toroidal coils having a diameter range of 17 mm to 19 mm through milk using a hand whipping motion, much like hand beating eggs. U.S. Pat. Nos. 5,580,169 and 5,780,087 each teach manually passing a screen attached to a plunger through milk in a vertical reciprocating motion. Each of the above mixtures may be pre-heated in a microwave oven.
For follow up reading, see Mathew Tekulsky et al., Making Your Own Gourmet Coffee Drinks: Espressos, Cappuccinos, Lattes, Mochas, and More! (Crown Pub., January 1993); Espresso Coffee: The Chemistry of Quality (Andrea Illy & Rinantonio Viani Eds., Academic Pr., October 1995); David C. Schomer, Espresso Coffee: Professional Techniques (Peanut Butter Pub., June 1996); Howard Schultz & Dori Jones Yang, Pour Your Heart into It: How Starbucks Built a Company One Cup at a Time (Hyperion, September 1997); Phillip Janssen, Espresso Quick Reference Guide (Eightball Books, September 1998); and Christie Katona & Thomas Katona, Cappuccino/Espresso: The Book of Beverages (Bristol Pub. Enterprises, March 1999).